Carbon nanotubes are one-dimensional nanomaterials consisting of cylinders of graphene. Depending on their diameter and helicity, carbon nanotubes consisting of a single graphene tubule, known as single-walled carbon nanotubes (SWCNTs), can behave either as metals or semiconductors whose band gap varies inversely with tube diameter. Multi-walled carbon nanotubes, which consist of multiple concentric graphene cylinders, typically possess much larger diameters than SWCNTs.
Accordingly, these carbon nanotubes demonstrate metallic or small band gap semiconducting behavior, and are mechanically stronger than SWCNTs. Developments in carbon nanotube synthesis have enabled the preferential production of multi-walled carbon nanotubes consisting of two walls. These double-walled carbon nanotubes (DWCNTs) can be synthesized using methods including chemical vapor deposition, electric arc discharge, and coalescence of chains of C60 inside SWCNTs. DWCNTs have garnered increasing attention for applications because their structure provides them with characteristics situated between those of SWCNTs and multi-walled carbon nanotubes having three or more walls (MWCNTs). Compared to SWCNTs and MWCNTs, DWCNTs have demonstrated better performance parameters in field-effect transistors, improved spatial resolution and longer scanning lifetimes as atomic force microscope (AFM) tips, and more desirable field emission characteristics.
Despite their promising applications, current methods of synthesizing DWCNTs also produce significant quantities of unwanted SWCNTs and MWCNTs. Multiple groups have succeeded in increasing the proportion of DWCNTs following synthesis using high temperature oxidation, which preferentially destroys the more thermally unstable SWCNT impurities. However, these oxidative treatments can degrade the electrical and optical properties of DWCNTs, and are ineffective at removing MWCNTs because DWCNTs and MWCNTs exhibit similar thermal stabilities.
As a result, more refined separation methods are required to provide nanotube populations that are highly enriched with DWCNTs. Furthermore, beyond separation by wall number, practical applications also require DWCNT materials that are enriched according to chirality, diameter and/or electronic type to ensure that their electrical and optical properties are uniform.